The advent of immobilization of enzymes on substrata of gelatin, glass, plastic or ceramic beads has greatly increased the potential for industrial production of organic chemicals, drugs, fuels and a host of other products. The vertical packed-bed reactors currently used in such reactions last a maximum of 750 hours due to microbial action and/or undesirable buildup of foreign substances on the bed.
Generated by living cells, enzymes are non-living proteins which serve as catalysts to convert the food of the cell into energy and a by-product which is discharged by the cell. They are true catalysts in that they induce a chemical change in a substance but are themselves unchanged. They are reaction specific in that a single enzyme reacts with a specific substance and produces a specific product. Thus enzymatic reactions are very specialized and produce highly pure products. Generally, enzymatic reactions take place over a narrow range of pH and temperature.
It is the reaction of enzymes which cause biological reactions to take place. Bacteria, yeasts, and fungi perform chemical transformations because of the enzymes they possess which enable them to digest a substance and produce a waste product, e.g., in the case of yeast, glucose is consumed and ethyl alcohol is produced as the waste product.
Recent developments in this field permit the separation of the enzyme from the cell itself and the attachment of that enzyme onto an inert substrate. The attachment may be of three types: adsorption on glass or ceramic beads; entrapment in a material such as starch or silica gel; and covalent attachment to glass or polymer. In this way, a highly concentrated enzyme is "immobilized" on an inert substance. This enzyme can then perform the desired catalysis in the absence of a living biological species.
This concept is now being used in commercial applications. Eight billion pounds per year of high-fructose corn syrup is being commercially produced using immobilized enzymes. Many other immobilized enzyme production processes are now entering the market place.
The reaction vessels in which these processes take place are either batch type reactors, or more recently, packed-bed reactors. This latter type consists of hollow columns packed with ceramic or glass beads. Feedstock enters the bottom of the column under pressure and product is extracted from the top. The life of the immobilized enzyme is limited by: crushing the beads under pressure; bacterial or microbial clogging of the surface of the beads, or other substances such as impurities or foreign substances in the feed adhering to the surface of the beads and reducing the reactivity of the enzyme. Typical life of the adsorbed immobilized-enzyme glass or ceramic bead ranges from tens of hours up to about 700 hours. Actually, reactivity is being constantly reduced furing the process. When it reaches the point of significant decrease in yield, the column is put out of service, emptied, and repacked with fresh immobilized enzyme beads, a relatively expensive and time consuming process which interrupts production.